1. Field of the Invention
This invention relates to helicopters, and more particularly to electronic control of stabilator surface and fail safe operation of positioning actuator systems.
2. Description of the Prior Art
Helicopters of the type using a single main rotor have a vertical tail rotor to prevent the aircraft from acquiring angular acceleration in a direction opposite to the rotation of the main rotor when in flight. Such helicopters are generally provided with a tail stabilizer which consists of a simple horizontal surface that provides aerodynamic pitch stability while the helicopter is in forward flight. Typically, the tail stabilizer may be mounted very high on the tail pylon so that the substantially vertical component main rotor downwash during hover and at low speeds does not react against the stabilizer surface, and typically such a single surface, which can be mounted on the pylon opposite to the tail rotor itself, suffices to provide adequate stability.
It has been known in helicopters of the prior art to provide a certain degree of control over the position of the stabilizer so that it may provide additional aerodynamic control to the attitude of the helicopter when in forward flight. A completely-manually controlled stabilator is described in U.S. Pat. No. 2,630,985; a stabilator working directly from the collective pitch lever is shown in U.S. Pat. No. 3,081,052; in a compound helicopter, the elevator segments of fixed stabilators are controlled by a stick in a fashion similar to a normal, fixed wing aircraft, as shown in U.S. Pat. No. 3,105,659; and, control over a variable angle of incidence stabilator in response to the cyclic pitch controls of a helicopter is illustrated in U.S. Pat. No. 3,721,404.
In the normal, single-main-rotor helicopter, which has a vertical stabilizing tail rotor, all of the lift to the helicopter is provided through the main rotor hub. Since the tail is a great distance away from the main hub, any weight near the tail is known to be difficult to lift, unless it is counterbalanced by comparable weight forward of the main rotor. In some circumstances, it becomes necessary to provide lift at the rear of a single-main-rotor helicopter. One solution is to cant the tail rotor so that some component of its thrust is in the downward direction, thereby providing lift directly at the extreme rear of the helicopter. However, having a canted tail rotor adds significant complexity to the achievement of aerodynamic stability of a helicopter, both in terms of the response of the helicopter to its own control surfaces, and in terms of response to external forces such as air pockets, gust inputs, and the like. If the design parameters of a helicopter include very stable high speed flight at extremely low altitudes, such as for a combat helicopter, a pair of stabilators, one on either side of the tail of the helicopter may be desirable. Additionally, control inputs to the stabilator which allow it to react to external inputs such as air density variations, wind gust inputs, and the like may be desirable.
In any helicopter having elevator-like controls, with respect to which the vertical position of the tail and/or the pitch attitude of the aircraft can be very rapidly changed during high speed flight by changes in the stabilator angle of incidence, great care must be taken to prevent any faults from allowing undesired angles of meaningful magnitude from occurring. For instance, if a stabilator were to erroneously receive a command for an extreme tail-surface-down position, the helicopter could pitch nose-down so quickly so as to reach an uncorrectable flight condition which could result in a crash. Therefore, it becomes most essential that any automatic helicopter stabilator operation be totally fail safe. This not only means that faults must be detectable, but they must be detectable and any undesirable response must be prevented in a very rapid fashion.